The present invention relates to the field of photolithographic techniques. More specifically, the invention relates to the detection of and compensation for focal plane location and planarity errors. Still more specifically, one embodiment of the present invention provides an improved method and apparatus for measuring the distance between the focal planes of the reticle and the substrate in a photolithography system which employs scanning or stepping of both the reticle and the substrate. Another embodiment of the present invention provides an improved method and apparatus for adjusting the focus of Wynn-Dyson or other unity magnification, telecentric, catadioptic lens system. Another embodiment of the present invention provides an improved method and apparatus for eliminating or modifying the sag or warp in a reticle or substrate in a photolithography system. The present invention will be particularly useful in photolithography systems that are used for printing on large substrates, printing from large reticles, or where the depth of focus is so small that even slight focus errors are important.
Currently there are a number of problems associated with holding reticles and substrates in high resolution photolithography systems so that they are flat and parallel to the degree required to maintain good focus. These problems are particularly pronounced in photolithography systems which rapidly process large area substrates using very large imaging fields as in the case of, for example, the manufacture of flat panel displays and multi-chip modules. Also problematic in this regard is the processing of devices such as microprocessor chips and dynamics memory (DRAM) chips which employs relatively large fields and lenses which have very shallow depth of focus.
One focus problem encountered with photolithography systems which move both the reticle and the substrate relative to the lens is the creation and maintenance of the proper gaps and parallelism between the reticle, substrate, and lens. Focus gauging and adjustment techniques exist for the more traditional lithography tools that have fixed lenses and reticles, but more and different techniques are needed for systems which move both the reticle and the substrate and for the new multiple axis scanners. Traditionally, the reticle stage (and therefore the reticle) is permanently fixed with respect to the lens, and the position of the substrate relative to the lens is sensed using, for example, reflective laser beams, capacitive techniques, or air gap sensing technology.
Another problem is encountered when adjusting the focus of a photolithography system. The traditional technique is to adjust the position of the substrate relative to the fixed lens and reticle. This is still a valid technique for some applications, but it would be very difficult for a system employing this technique to adequately compensate for realtime focus variations in a high-speed scanning application such as those used to process large area substrates.
Another focus problem involves keeping the reticle and substrate planes parallel. Keeping the substrate flat is relatively easy. It can be rigidly attached over its entire surface to a flat substrate chuck with vacuum. However, the reticle must be supported only on its perimeter to allow a clear aperture over the majority of its area through which light may be projected. This type of support allows any natural warp or stress in the reticle to be expressed as a focal plane error. Mounting the reticle on a reticle chuck with a predominately horizontal attitude exacerbates the problem by inducing sag of the reticle due to gravity. Currently, no satisfactory techniques exist which prevent or compensate for this problem.
From the foregoing, it will be understood that improved techniques for dealing with focus problems are needed, particularly for multiple axis scanning systems, large area substrate processing, and systems employing lenses with shallow depth of focus.